46 



Dr. Foote and Dr. Mobler on Ionization and 



upon photoelectrical^ by ordinary light. The work of Tate 

 and Foote* proves that light of wave-length X=7685 cannot 

 produce ionization because this radiation is emitted during a 

 resonance collision, and the total current-curves showed no 

 trace of ionization at this point. We should expect the 

 limiting long wave-length of photo-sensibility to be X=2857, 

 and believe that the experimental evidence so far published 

 is not sufficient to show that ionization of potassium vapour 

 can be produced by ordinary light. 



Determination of h. 



Since the ionization and resonance potentials of electrons 

 in metallic vapours obey the quantum relation eY = hv, 

 this method is one which lends itself for a determination 

 of /i, Planck's constant of action. If we use the value 

 of <? = 4*774 . 10~ 10 determined by Millikan we obtain 

 /i = 0-5308A,Y. 10" 30 erg sec. where V is the value of the 

 ionization or resonance potential in volts and X is the wave- 

 length in Angstrom units corresponding to the frequency 

 v=l'5 S or k=1"5 S — 2p 2 . Table VI. presents the data so 

 far accumulated : — 



Table VI. 

 Determination of h. 



Element. A (1*5 S). 



Cd 1378-69 



Na 241 2-83 (fl) 



K 2856-65 (a) 



Zn 1319-95 



Hg 118796 



Mg 1621-7 



Tl ? 



\(l-5S-2p 2 ). 



Cd 3260-17 



Ka 5893- (a) 



K , 7685- (a) 



Zn 3075-99 



Hg 2536-72 



Mg 457138 



Tl 11513-220) 



(a) Properly danoted by T5s or V5s—2p, except for thallium, where 2p. 2 



refers to the shorter wave-length of the doublet, 



(b) Tate and Foote. 



(c) Foote and Mohler. 



(d) Franck and Hertz. 



(<?) Tate, Davis and Goucher, and others. 



Observed 

 Ionization. 



Observer. 



Observed 

 h . 10 27 . 



8-92 volts. 



b 



6-53 



5-13 



b 



6-57 



4-1 



b 



6 22 



9-5 



b 



6-66 



10-35 



e 



6-53 



7-75 



c 



6-67 



73 



c 





Observed 







Resonance. 







3-88 



b 



6-71 



2-12 



b 



663 



1-55 



b 



632 



41 



b 



6-70 



4-9 



d, e 



6-60 



2-65 



c 



6-43 



1-07 



c 



6-54 





Mean 



655 



Phil. Mag. I. c. 



